http://en.wikipedia.org/wiki/Anaphylatoxin
Anaphylatoxins, or anaphylotoxins, are fragments ( C3a, C4a and C5a) that are produced as part of the activation of the complement system
Vasodilation- Vasodilation is the process by which the vasculature (artery/arterioles/capillaries/venules/veins) dilate , permitting more quantity of blood to flow through. This happens under the stimulus of certain substances either secreted locally by cells or nervous stimulation or by the action of certain drugs(tablets/injection) we take.
vasodilators eg: histammines, nitrates, ....etc

Anaphylatoxins are able to trigger degranulation (release of substances) of endothelial cells, mast cells or phagocytes, which produce a local inflammatory response. If the degranulation is widespread, it can cause a shock-like syndrome similar to that of an allergic reaction.
Anaphylatoxins indirectly mediate:
smooth muscle cells contraction, for example bronchospasms
increase in the permeability of blood capillaries
chemotaxis — receptor-mediated movement of leukocytes in the direction of the increasing concentration of anaphylatoxins